This application claims the priority of Japanese Patent Application No. 11-021382 filed on Jan. 29, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a luminous flux variable type objective lens which is composed of a single lens adapted to change the luminous flux diameter according to its wavelength in use, and an optical apparatus using the same; and, in particular, to a luminous flux variable type objective lens capable of reproducing signals from each of a plurality of optical disks for which different wavelengths of irradiation light are used, and an optical apparatus, such as an optical pickup apparatus, using the same.
2. Description of the Prior Art
Optical disks have been in use as various kinds of recording media due to their high recording density, large capacity, excellent random accessibility, and the like.
Hence, there have been known a number of kinds of optical disks such as CD (compact disk), CD-R (recordable compact disk), DVD (digital video disk), LD (laser disk), and the like according to their use or field of application.
Since the optical pickup apparatus have substantially the same configuration for various kinds of optical disks, if a so-called compatible player capable of reproducing signals from various kinds of optical disks is constructed by use of a single pickup apparatus, it will be convenient in that it is unnecessary to prepare various kinds of players for the respective kinds of optical disks.
Meanwhile, the value of numerical aperture NA for each recording medium is defined by a standard. For example, the numerical aperture for DVD is 0.6, thus needing brightness, whereas that of CD-R is 0.45, thus suppressing brightness.
Therefore, if the same objective lens is employed for the recording/reproducing of both of the above-mentioned disks, it will be necessary to switch the numerical aperture of the pickup optical system between their recording/reproducing operations.
The switching between larger and smaller numerical apertures is generally effected by changing the luminous flux diameter. For example, a liquid crystal shutter is driven in synchronization with the timing at which the above-mentioned recording/reproducing operations for the disks are changed over from each other. In this case, however, since the liquid crystal shutter is disposed on the optical path separately from the objective lens, it is contrary to the demand for making the pickup optical system compact. Also, its optical adjustment becomes troublesome. Further, since the light passable through the liquid crystal shutter is required to be polarized, the designing has to be done in view of the direction of vibration of this irradiation light, whereby the degree of freedom in design may be restricted.
Meanwhile, as for the above-mentioned two types of optical recording media, visible light having a wavelength of about 635 nm is used for DVD in order to improve its recording density, whereas near infrared light having a wavelength of about 780 nm is required to be used for CD-R since it is not sensitive to light in the visible region. In view of this fact, there has recently been disclosed a technique in which the difference in wavelength between irradiation light beams is utilized for changing the luminous flux diameters of these two irradiation light beams.
Namely, as one utilizing such a difference between wavelengths of irradiation light in use, there has been known a single lens (Japanese Unexamined Patent Publication No. 9-145994) in which a plurality of regions having curvatures different from each other are concentrically disposed in a lens surface so as to be able to emit luminous fluxes having numerical apertures different from each other.
In such a technique, however, since the lens surface form is complicated, the die for molding the lens becomes expensive, whereby the initial cost before making the lens would increase.
In view of the foregoing circumstances, it is an object of the present invention to provide a luminous flux variable type objective lens capable of changing the luminous flux diameter of irradiation light according to the wavelength in use without considerably complicating the lens surface form and without increasing the cost of manufacture of the lens molding die, and an optical apparatus using the same.
The luminous flux diameter variable type objective lens in accordance with the present invention is a luminous flux diameter variable type objective lens, disposed within a luminous flux, having a wavelength selectivity;
the objective lens having a first region constituting a center part and a second region positioned outside the center part, the first region having no diffracting action or reflecting action with respect to two light beams having respective wavelengths xcex1, xcex2 different from each other, the second region being formed with a diffracting portion or reflecting portion having a diffracting action or reflecting action with respect to only one of the two light beams having respective wavelengths xcex1, xcex2.
The diffracting portion may comprise a diffraction grating in which a series of protrusions each having a rectangular cross section are arranged in one direction with a predetermined pitch therebetween.
Alternatively, the diffracting portion may comprise a diffraction grating having a stepped cross section.
The diffraction grating may be made by deposition of titanium dioxide.
The reflecting portion may be constituted by a dichroic reflective film coated on a surface of the lens.
The optical apparatus in accordance with the present invention comprises the luminous flux diameter variable type objective lens in accordance with the present invention.
In the luminous flux variable diameter type objective lens and optical apparatus in accordance with the present invention, a diffracting portion or reflecting portion having a wavelength selectivity is disposed in a peripheral region of the lens, so that only the light whose luminous flux diameter is to be narrowed is not converged in a predetermined converging direction in this diffracting portion or reflecting portion.
Therefore, of the light irradiating the luminous flux diameter variable type objective lens, a wavelength component whose luminous flux diameter is not needed to be narrowed is transmitted therethrough with its original luminous flux diameter, so as to keep the NA of the optical system at a greater value; whereas, as for a wavelength component whose luminous flux diameter is to be narrowed, only its luminous flux portion irradiating the center region of the lens is transmitted therethrough as it is, so as to narrow the luminous flux diameter, thus causing the NA of the optical system to become a smaller value.
As a consequence, the present invention achieved in view of the fact that wavelengths of light employed in individual optical recording media are different from each other is quite effectively applicable to a common optical pickup apparatus for DVD and CD-R, in which the size of NA is needed to be switched. Namely, while substantially utilizing the configuration of the conventional optical pickup apparatus as it is, the luminous flux diameter variable type objective lens in accordance with the present invention having a simple configuration may simply be inserted therein in place of a normal objective lens, so as to switch the NA to the one corresponding to an optical recording medium. Since the die for molding the lens can be made easier than in the above-mentioned prior art, the cost of manufacture can be cut down, whereby the practical value of the present invention is quite high.